In gas turbine engines, air discharged from a compressor section and fuel introduced from a fuel supply are mixed together and burned in a combustion section. The products of combustion are harnessed and directed through a turbine section, where they expand and turn a central rotor. For land-based gas turbine engines, the rotor so turned typically powers an electric generator to generate electricity.
A variety of combustor designs exist, with different designs being selected for suitability with a given engine and for achieving desired performance characteristics. One popular combustor design, known as a can-annular type design, comprises in each of a plurality of arranged “cans” a centralized pilot burner and a number of main fuel/air mixing apparatuses. The main fuel/air mixing apparatuses are arranged circumferentially around the pilot burner, and each such apparatus, during operation, produces a fuel/air mixture that is combusted. In order to ensure optimum performance, it is generally preferable that a respective fuel-and-air mixture is well mixed to avoid localized, fuel-rich regions. As a result, efforts have been made to produce combustors with essentially uniform distributions of fuel and air. Swirler elements, for example, are often used to produce a stream of fuel and air in which air and injected fuel are evenly mixed.
One objective in design and operation of gas turbine combustors is the stability of the flame and, related to that, the prevention of flashbacks. A flashback occurs when flame travels upstream from the combustion zone in the combustion chamber and approaches, contacts, and/or attaches to, an upstream component. Although a stable but lean mixture is desired for fuel efficiency and for environmentally acceptable emissions, a flashback may occur more frequently with a lean mixture, particularly during unstable operation. For instance, the flame in the combustion chamber may progress backwards and rest upon, for a period, a base plate which defines the upstream end of the combustion chamber. Less frequently, the flame may flash back into a fuel/air mixing apparatus, damaging components that mix the fuel with the air.
A multitude of factors and operating conditions provide for reliable, efficient and clean operation of the gas turbine combustor during ongoing operation. Not only is the fuel/air mixture important, but also relevant to gas turbine operation are the shape of the combustion area, the arrangement of assemblies that provide fuel, and the length of the combustor that provides varying degrees of mixing. Given the efficiency and emissions criteria, the operation of gas turbines requires a balancing of design and operational approaches to maintain efficiency, to meet emission standards, and to avoid damage due to undesired flashback occurrences.
The fuel/air mixing apparatus, and how it operates in relationship to other components, is one of the key factors in proper operation of current gas turbines. A common type of fuel/air mixing apparatus is known as a main swirler assembly. A main swirler assembly is comprised in part of a substantially hollow inner body that comprises stationary flow conditioning members (common forms of which also are referred to as vanes) that create a turbulent flow. Fuel from a fuel nozzle is added before or into this turbulent air stream and mixes to a desired degree within a period of time and space so that the air and fuel are well mixed upon combustion in the downstream combustion chamber. Also, in typical arrangements, a main swirler assembly also is comprised of an outer downstream element known as a sleeve. A sleeve (referred to in some references as an “annulus casting”) surrounds a downstream section of the inner body, forming a channel for air flow known as the flashback annulus. In a typical arrangement, a quantity, such as eight, of swirler assemblies are arranged circumferentially around the central pilot burner. The pilot burner typically burns a relatively richer mixture than is provided by the radially arranged swirler assemblies.
Examples of approaches to reach a balance among the needs to reduce flashbacks, maintain reasonable initial costs, maintain operating efficiency, and reduce downtime and costs due to component failure, are provided in the following patents and applications: U.S. Pat. No. 6,705,087, issued Mar. 16, 2004 to R. Ohri and David M. Parker, U.S. patent application Ser. No. 10/984,526, filed Nov. 9, 2004, and entitled “An Extended Flashback Annulus”, and U.S. patent application Ser. No. 11/051,799, filed Feb. 4, 2005, and entitled, Can-Annular Turbine Combustors Comprising Swirler Assembly And Base Plate Arrangements, And Combinations”. These and all other patents, patent applications, patent publications, and other publications referenced herein are hereby incorporated by reference in this application in order to more fully describe the state of the art to which the present invention pertains, to provide such teachings as are generally known to those skilled in the art, and to provide teachings specific to embodiments of the present invention that utilize combinations of features that include one or more features and/or components described in the referenced patent applications.
Despite the advances in the art, there remains a need to provide more suitable designs related to combustors and main swirler assemblies to better solve flashback and other issues during gas turbine operation. This, in part, is due to the fact that the combustion dynamics of full-scale gas turbine engine combustors do not predictably or reliably scale from smaller model systems, which means that there is a greater degree of unpredictability for multi-feature combustors.